This invention relates generally to improved apparatus and methods for deposition processes in the manufacture of semiconductor materials.
Chemical vapor deposition (CVD) processes have long been used to form thin layers on substrates (and wafers) by sequential layer deposition by thermal reaction or decomposition of gaseous material (reactants) at the substrate surface. In a specific type of deposition process, atomic layer epitaxy (ALE), sequential monolayers are deposited on a substrate surface by alternate exposure to chemical reactants. Repeated exposure cycles of the substrate to reactant gases builds the desired layer structure. ALE techniques are described for example in M. Ritala et al. (1998) J. ElectroChemical Society 145:2914; H. Shrinriki et al. (1998) J. ElectroChemical Society 145:3247 and J. L. Vossen et al. “Thin Film Deposition Processes II” (Academic Press, California 1991). The terms ALE and “atomic layer chemical vapor desorption,” ALCVD™ are considered equivalent in this disclosure.
A variety of apparatus have been employed for layer deposition processes. A basic system includes a reactant chamber, a substrate holder with a heater, a gas flow system including gas inlets for providing reactants to the substrate surface and an exhaust system for removing used gases.
Deposition apparatus are configured for batch processing of multiple substrates or single-substrate processing. Single-substrate processing is presently more preferred for larger substrates to improve uniformity of deposition. Horizontal gas flow reactors, such as described in Ozias U.S. Pat. Nos. 4,846,102; 5,096,534; and 5,244,694 which concentrate reactant gas flow at the substrate surface and particularly those equipped with gas manifolds that create uniform reactant gas velocity profiles (Hawkins et al. U.S. Pat. No. 5,221,556) provide efficient uniform processing of large single substrates.
ALCVD™ deposition processes, particularly as practiced in horizontal flow reactors for single wafer processing, are performed in a step-wise manner, in which a first reactant is introduced into the reaction chamber through a gas inlet or manifold to form a deposited layer on the substrate. Excess reactant gas is then evacuated from the reaction chamber in a pump-down step (see for example, Sherman, U.S. Pat. No. 5,916,365). Optionally, an inert purge gas is flowed through the gas inlet to remove residual reactant. After the pump down, a second reactant is introduced into the chamber to react with the deposited reactant to form the desired substrate layer. Excess reactant is then removed in another pump-down step. Layers are added to the substrate surface by sequential additions of various reactant gases with intervening chamber pump-down. Step-wise processing with chamber evacuation is employed to separate reactant gases and minimize reaction of these gases in the gas phase or in parts of the reaction chamber other than on the substrate to avoid formation of particles that are detrimental to substrate processing and to avoid depletion of reactants. Intervening chamber evacuation steps represent a significant portion of the time required for processing a substrate, in most cases exceeding 50%. A significant decrease in process time leading to a significant decrease in manufacturing cost could be achieved by eliminating the chamber evacuation steps.
Suntola et al. U.S. Pat. No. 4,389,973 and U.S. Pat. No. 5,711,811 describe apparatus for ALE of a substrate in which sequentially applied reactant gases are separated by inert gas phase diffusion barriers. For example, timed pulses of reactant gases are transported into a reaction chamber to interact with the substrate in a continuous flow of carrier gas passing through the chamber. Reactant gases are thus applied separately to the substrate in a continuous gas flow without need of intervening chamber evacuations. U.S. Pat. Nos. 4,747,367 and 4,761,269 (Crystal Specialties) describe chemical vapor deposition methods in which a constant flow and pressure of gas is maintained on sequentially pulsing of reactant gases into a neutral carrier gas stream. When a reactant gas is switched into or out of the carrier gas flow, the carrier gas flow is decreased or increased, respectively, to maintain constant flow and pressure in the reaction chamber. It is important to note that these techniques work only at sufficiently high pressures (>10 torr), where diffusion in the gas phase is sufficiently low.
It is known in substrate deposition processes to employ excited species, particularly radicals, to react with and/or decompose chemical species at the substrate surface to form the deposited layer. In processes using activated species, the apparatus is provided with a device for excitation. Radicals can, for example, be generated (along with ions) by application of RF or microwave energy to form a plasma. A number of alternative methods for formation of radicals are known in the art, including, for example, thermal decomposition and photolysis.
Reactive species, including radicals, can be generated in situ in the reactant chamber at or near the substrate surface or generated remotely and subsequently carried, e.g., by gas flow, to the reaction chamber. See, U.S. Pat. Nos. 4,664,937, 4,615,905 and 4,517,223 for in situ radical generation and U.S. Pat. No. 5,489,362 for remote radical generation. Remote radical generation allows exclusion of potentially undesirable reactive species (e.g., ions) that may be detrimental to substrate processing. However, remote radical generation techniques should provide sufficient radical densities at the substrate surface, notwithstanding the significant losses that can occur on transport of the radical to the reaction chamber. Radical losses are generally severe at higher pressure (>10 Torr), thus precluding the use of higher pressure to separate the reactants in an ALE process.